Microstrip antennas of many types are now well-known in the art. Briefly, microstrip antenna radiators comprise resonantly dimensioned conductive surfaces disposed less than about one-tenth of a wavelength above a more extensive underlying conductive ground plane. The radiator elements may be spaced above the ground plane by an intermediate dielectric layer or by suitable mechanical standoff posts or the like. In some forms (especially at higher frequencies, such as UHF), the microstrip radiators and interconnecting microstrip RF feedline structures are formed by photochemical etching techniques (like those used to form printed circuits) on one side of a doubly clad dielectric sheet, with the other side of the sheet providing at least part of the underlying ground plane or conductive reference surface.
Microstrip radiators of many types have become quite popular due to several desirable electrical and mechanical characteristics. However, microstrip radiators naturally tend to have relatively narrow bandwidths (e.g., on the order of 2-5% or so). This natural characteristic sometimes presents a considerable disadvantage and disincentive to the use of microstrip antenna systems.
For example, there is considerable demand for antennas in the L-band frequency range which covers both of the global positioning satellite (GPS) frequencies L1 (1575 MHz) and L2 (1227 MHz). It may also be desirable to include the L3 frequency (1381 MHz) to enable the system to be used in either a global antenna system (GAS) or in G/AIT IONDS program. As may be appreciated, if a single antenna system is to cover both bands L1 and L2, the required bandwidth is on the order of at least 25% (e.g., .DELTA.F divided by the midpoint frequency).
Although microstrip radiating elements have many characteristics (e.g., physical ruggedness, low cost, and small size) that might make them attractive for use in such a medium bandwidth situation, available operating bandwidths for a given microstrip antenna radiator have typically been much less than 25%--even when "broadbanded" by use of prior art techniques.
Various ways to "broadband" a microstrip antenna assembly are known. For example, related copending, commonly-assigned application Ser. No. 864,854 of Paschen filed May 20, 1986 discloses a microstrip antenna which is broadbanded by optimizing the inductive and capacitive reactances of the antenna feedline.
Previous attempts at producing a broadband microstrip antenna array element generally followed two basic approaches: (1) the thick substrate microstrip patch; and (2) the single capacitively-coupled resonator radiator.
The thick substrate microstrip patch 10 (shown in prior art FIG. 1) includes a relatively thick dielectric substrate 12 which separates the patch ground plane 14 from the radiating patch 16 (and thus defines a cavity of relatively large dimension between the two patches). A coaxial feedline connection 18 has its ground conductor connected to ground plane patch 14 and its center conductor connected to patch feed pin(s) 20. Feed pin(s) 20 pass through substrate 12 and conduct RF between connection 18 and radiating patch 16.
The thick substrate patch shown in FIG. 1 has a practical maximum bandwidth of 12%-15% at 2.0:1 VSWR (voltage standing wave ratio). In order to achieve this bandwidth performance, however, two feed pins 20a and 20b are required to ensure cancellation of the cross-polarized component and maximize radiation efficiency. Inclusion of these feed pins 20 (and associated required phasing circuitry 22) severely limits the practical use of the thick substrate patch design in antenna arrays, since the fabrication process is complicated, and structural strength and reliability are compromised.
Concerns over reliability and production cost rule out the use of the feedthroughs necessary for thick substrate elements, at least for antenna structures which are to be mass produced and/or used in harsh environments or critical applications. Dual linear or circularly polarized operation of thick substrate elements aggravates these cost and reliability problems, since an orthogonal pair of feed connections are required--resulting in a total of four feed pins per patch.
The single capacitively coupled element 30 shown in prior art FIG. 2 eliminates the need for direct feedthrough connections. The driven patch 32 is fed by microstrip circuitry (not shown) printed on the driver substrate 34 and directly connected to the driven patch. Energy radiated by driven patch 32 excites a parasitic element 36 separated from the driven patch by a foam dielectric spacer 38. Parasitic element 36 and driven patch 32 have slightly different resonant frequencies--resulting in a broadbanding effect.
The structure shown in FIG. 2 has a bandwidth which is comparable to that of the structure shown in FIG. 1, is very easy to fabricate (for example, the three layers may be laminated together), and is also easily adapted to varying polarization requirements. Unfortunately, the maximum bandwidth of the FIG. 2 structure is only about 14% at 2:1 VSWR. While this bandwidth is sufficient for certain applications, greater bandwidth is often required.
It is possible to increase the bandwidth of the structure shown in FIG. 2 to up to about 18% bandwidth by providing 1/2 wavelength matching stubs. Unfortunately, the matching circuitry takes up a substantial amount of substrate real estate, increasing the size of the antenna structure. Moreover, the average VSWR of such a structure has been calculated and experimentally verified to be about 1.9:1--which is too high for the output stages of many RF transceivers and also results in inefficiency due to excessive transmission line return loss.
Some non-exhaustive examples-of prior art techniques for achieving a broadened bandwidth microstrip antenna are illustrated by the following prior issued U.S. patents:
U.S. Pat. No. Re 29,911--Munson et al (1979) PA1 U.S. Pat. No. 4,070,676--Sanford (1978) PA1 U.S. Pat. No. 4,180,817--Sanford (1979) PA1 U.S. Pat. No. 4,131,893--Munson et al (1978) PA1 U.S. Pat. No. 4,160,976--Conroy (1979) PA1 U.S. Pat. No. 4,259,670--Schiavone (1981) PA1 U.S. Pat. No. 4,320,401--Schiavone (1982) PA1 U.S. Pat. No. 4,329,689--Yee (1982) PA1 U.S. Pat. No. 4,401,988--Kaloi (1983) PA1 U.S. Pat. No. 4,445,122--Pues (1984) PA1 U.S. Pat. No. 4,477,813--Weiss (1984) PA1 U.S. Pat. No. 4,529,987--Bhartia et al (1985)
See also Sanford, "Advanced Microstrip Antenna Development", Volume I, Technology Studies For Aircraft Phased Arrays, Report No. FAA-FM-80-11-Vol-1; TSC-FAA-80-15-Vol-1 (June, 1981).
As discussed in some of the prior art references cited above--particularly in commonly-assigned U.S. Pat. No. 4,070,676 to Sanford--the typical 2-5% natural bandwidth of a microstrip radiator can be increased somewhat by stacking multiple radiators of various sizes above the ground plane parallel to one another and parallel to the ground plane. In one embodiment disclosed in the Sanford patent (and shown in prior art FIG. 3 of the subject application), elements 40,42 of different sizes are spaced apart from the ground plane surface 44 (and from one another) by layers of dielectric material 46,48. The largest element 40 is located nearest the ground plane, with successively smaller elements being stacked in the order of their resonant frequencies.
The topmost of Sanford's elements (42) is driven with a conventional microstrip feedline 50, while element 40 disposed between the topmost element and the ground plane remains passive. Mutual coupling of energy between the resonant and non-resonant elements causes the parasitic elements to act as extensions of the ground plane and/or radio frequency feed means. The resulting compact multiple resonant radiator exhibits a potentially large number of multiple resonances with very little degradation of efficiency or change in radiation pattern.
Others have also designed stacked microstrip antenna structures. For example, the Kaloi patent discloses a coupled multilayer microstrip antenna having upper and lower elements tuned to the same frequency in an attempt to provide enhanced radiation at angles closer to the ground plane.
The Yee patent discloses a broadband stacked antenna structure having three discoid elements stacked above a ground plane in order of decreasing size. A coaxial cable center conductor is electrically connected to the top conducting plane. Yee also provides openings through his intermediate elements (supposedly to increase coupling of energy between the stacked elements). The Yee patent claims that the bandwidth of this structure is "at least as great as 6%, and possibly higher, even up to 10%." As can be appreciated, this bandwidth is insufficient for many applications.
It would be highly desirable to produce a rugged, efficient, easy to fabricate, broadband, dual linearly polarized, microstrip antenna array element that does not require a separate impedance matching circuit or feedthrough connections between layers, and yet provides a 2.0:1 VSWR bandwidth of at least 18%.